1. Field of the Invention
The present invention relates to an X-ray computed tomography (CT) method and system for producing sectional images of local region and panoramic images of an object by radiating specific conical X-ray beam to the object to be examined.
2. Description of the Prior Art
A method of obtaining a sectional image known as X-ray computer tomography (CT) has been widely used in medical practice as for diagnosis. This method involves radiating X-rays on the object from all around, followed by analyzing three-dimensional absorption coefficient distribution information on the X-rayed object from the projection data by the Radon theory as widely known, thereby obtaining a sectional image of object.
The prior art X-ray CT is a technique in which a fan-shaped X-ray beam relatively wide in the direction of rotation and thin is radiated on the object from around at a depth and this is repeated at different depths.
Accordingly, in the event that only a part inside the object is to be put to tomography examination, the fan-shaped wide X-ray beam is radiated over the whole object to obtain an absorption coefficient distribution information from which the coefficient distribution information on that part or region is taken out for analysis. That is, the object is exposed to a substantial dose of the X-ray beam.
In addition, it takes long to radiograph and analyze the test data. In the light of high dose of radiation, the CT examination is limited to some once a year.
In the prior art panoramic X-ray system, the X-ray beam is scanned over the dental arch from behind roughly crosswise in relation to the dental arch. The rotary arm is so designed that the X-ray generator is moved continually around three different centers a, b and c in the front tooth area of the dental arch, on the right and left of the molar tooth area as shown in FIGS. 24(a) to (c). That requires a complicated mechanism and control of the rotary arm (not shown) to rotate the X-ray generator 101.
FIG. 25 shows loci of the rotary arm of another prior art panoramic X-ray system during the X-ray examination. The letter Lo designates the axis of symmetry and is the median line of the axial symmetry of a dental arch. The letter L is the X-ray beam bundle roughly perpendicular to the radiation area of the dental arch. The letter La is an envelope line formed by the bundle L of the X-ray beams. In this case, too, the rotary arm has to be so moved as to form an envelope line as shown in the figure. That makes moving mechanism and control complicated.